1. Field of the Invention
The present invention relates to a method for improving the optical coupling of the edge-coupled photodetector, especially a high-speed semiconductor edge coupled photodetectors which usually have a coupling aperture much smaller than the output spot-size (mode-field diameter) of the coupler.
2. Description of the Prior Art
Semiconductor photodetectors are indispensable in fiberoptic communications for receiving transmitted information and, as required data capacity increases, it is essential for the detectors to achieve high-speed data operation with high receiving efficiency. As shown in FIG. 1(a), a conventional semiconductor photodetector has a layer structure wherein P+ type and N+ type semiconductor layers (111 and 113) sandwich therebetween a low- or un-doped optical absorption layer 112. This photodetector is generally applied with a reverse bias between the P+ and N+-type semiconductor layers to deplete the optical absorption layer of carriers, and uses the high electric field generated in the depleted region to collect the photo-generated carriers.
Conventional surface-coupled photodetectors, which receive the incoming light 114 from the top of the device, have their optical absorption path parallel to and overlapping with carrier transit path. Reduction of the carrier transit path, though enhances the operating speed (if the detector bandwidth is transit-time-limited), results in lower absorption efficiency and thus degrades detector sensitivity. In other words, the surface-coupled photodetectors can not have the maximum bandwidth and maximum quantum efficiency simultaneously. More specifically, there exists a maximum value for the bandwidth-efficiency product of surface-coupled photodetectors. For instance, 30 GHz is a typical bandwidth-efficiency product value for InP-based long-wavelength surface-coupled photodetectors.
On the other hand, the edge-coupled photodetectors, which receive the incoming light 115 from the edge of the device, have their optical absorption path xcex94Z and carrier transit path xcex94X perpendicular to each other, therefore the two path lengths can be independently tuned. Ideally, the edge-coupled photodetectors can have their highest bandwidth along with the highest quantum efficiency. However, in practical case, as the absorption layer thickness xcex94X decreases for shortening the carrier transit time, the optical coupling using conventional waveguide device, such as the optical fiber, gets harder since the epi-structure-defined coupling aperture shrinks accordingly. For instance, xcex94X must be smaller than 1 xcexcM for 30 GHz detector bandwidth, while typically the single-mode fiber has a beam diameter larger than 5 xcexcm. It is therefore more difficult to adapt the coupling aperture of the photodetector to the mode field diameter of an optical fiber, thereby causing a problem of coupling loss therebetween.
As shown in FIG. 1(a), the coupling aperture of edge-coupled photodetectors locates at the edge side of the device; therefore its effective size is determined by the cross section of the semiconductor layer structure and can be approximately represented by xcex94Xxcex94Y Using an optical fiber as a coupler for direct coupling the light 115 into the effective aperture of a high-speed photodetector for optical absorption in the thin absorption layer 112, one can achieve the high coupling efficiency only if the optical spot size is adequately small and the optical alignment, especially in the direction perpendicular to the device substrate, is accurate. For this coupling issue, a photodetector with a waveguide structure, which enlarges the effective coupling aperture, has been disclosed both in U.S. Pat. No. 5,991,473 and 5,998,851, and is schematically shown in FIG. 1(b). Through this waveguide-forming layer structure, the effective coupling aperture (mode-field diameter) can be increased approximately from, for example, 0.41 xcexcm to 3 xcexcm, and thus the detector efficiency can be significantly promoted. However, the total layer thickness of the epi-structure is typically larger than 7 xcexcm, which requires the epitaxial growth time at least double than the conventional detector structure (FIG. 1(a)) for additional thick cladding layers 114 and 115. The photodetector with tapered absorption layer proposed in U.S. Pat. No. 5,998,851 even requires a re-growth process, which introduces almost doubled epitaxy cost, and reproducibility and reliability issues. These additional cost and issues also happen in the waveguide-integrated photodetectors disclosed in U.S. Pat. Nos. 4,835,575, 5,285,514, 5,521,994, and 6,498,337. Besides, in addition to direct coupling, these waveguide-type or waveguide-integrated photodetector utilizes indirect evanescent coupling, which requires at least 20-30 xcexcm detector length for the propagating light to be absorbed and therefore inevitably introduces additional junction capacitance. Larger junction capacitance results in lower device bandwidth.
According to the definition of the active pn junction region, the edge-coupled photodetectors can be divided into two categories: junction-mesa type as shown in FIG. 1(a) and selective-area-diffused (SAD) mesa type as shown in FIG. 1(c). The junction mesa type has a pn junction formed during the layer epitaxy and has a junction area defined by photolithography and etching process. The SAD-mesa type has a pn junction formed by localized diffusion process, which in the meantime defines the junction area. Due to that the depleted absorption region is sealed inside, the SAD-mesa type photodetector is generally considered more reliable than the junction-mesa type photodetector. Let us consider an exclusive problem encountered by the SAD-mesa type photodetector. As shown in FIG. 1(c), the photodetector is composed by a highly n-type doped wide-bandgap semiconductor layer 113, a low- or un-doped narrow-bandgap semiconductor absorption layer 112, and a highly p-type doped wide-bandgap semiconductor region 111a, which defines the active region of the photodetector and is formed by the diffusion process, surrounded by a low- or un-doped wide-bandgap semiconductor region 111b. The low- or un-doped region 111c with an appropriate width xcex94t left in front of the active region effectively protects the active region from the outer environment. The regions outside the borders defined by the diffusion area with xcex94t outward extensions (i.e., the border defined by the thick dash lines) are out of the reach of the biasing field, and therefore are regarded as the inactive regions. Such regions exist exclusively only in the SAD-mesa type photodetectors. During optical coupling, the misalignment results in the coupling loss, which is no exception to the junction-mesa type photodetectors. However, besides coupling loss, lateral misalignment in Y direction can result in optical absorption in the inactive absorption regions of the SAD-mesa type photodetectors. Those photons being absorbed give up the energy which excite the electrons in the valence band and consequently generate electron/hole pairs. The electron/hole pairs generated in the inactive absorption regions either recombine in a short periods of time or slowly diffuse into the biased active region, at which they are then accelerated by the biasing field toward the respective electrodes. These xe2x80x9cslowxe2x80x9d carriers, relative to the xe2x80x9cfastxe2x80x9d carriers generated and drifting in the biased active region, result in signal tailing and deteriorate the device bandwidth. In summary, besides the coupling loss, there exist other issues relating to the misalignment, such as bandwidth reduction and linearity degradation caused by the slow carriers.
It is an object of the present invention to provide a light funnel integrated right in front of the coupling aperture of the edge-coupled photodetector for enlarging the effective coupling aperture and thereby enhancing the optical coupling efficiency. The funnel is preferably made of dielectric materials, such as silicon dioxide or polymer, and itself provides the optical confinement in the directions perpendicular to the optical axis that is referred as the Z-direction hereinafter. In the direction perpendicular both to the optical axis and the device substrate, hereinafter referred as the X-direction, the funnel effect is realized by utilizing either a wet etched, crystallographically defined semiconductor slope or a dry etched, resist-profile-defined semiconductor slope covered first by a highly reflective metal film and then a planarized dielectric layer. The funnel width in the X-direction therefore shrinks gradually along the +Z-axis. In the direction perpendicular to the optical axis while parallel to the device substrate, hereinafter referred as the Y-direction, the funnel effect is realized by utilizing the photolithography and the etching process to define a gradually shrinking funnel width along the +Z-axis. In appearance, the funnel""s top plane is essentially parallel to the device substrate, its bottom plane has an adequate included angle with the device substrate or has a tapering profile with an adequate curve function, such as parabolic taper or S-Bend cosine taper, and the two side planes perpendicular to the device substrate have adequate included angles with the optical axis or have tapering profiles with adequate curve functions, such as parabolic taper or S-Bend cosine taper. The funnel exit is just the optical entrance of the detector. The funnel internals can be partially or fully metallized for total mirror reflection. The lightwave entering the funnel converged in the X direction through the metallic mirror or total internal reflection and guided in the Y direction also through the metallic mirror or total internal reflection.
It is another object of the present invention to provide a light funnel integrated right in front of the coupling aperture of the edge-coupled photodetector for coupling the light into the active absorption region of the photodetector more uniformly and thereby reducing the input optical density. Reducing the optical density results in less carrier density that produces less space-charge effect and improves the device linearity.
It is a further object of the present invention to provide a light funnel integrated right in front of the coupling aperture of the edge-coupled photodetector. The funnel allows a larger optical spot size or a larger alignment tolerance for optical coupling. The incident lightwave is funneled into the active region without spilling into the inactive absorption region. Therefore no slow carrier generates and the device can have high quantum efficiency without sacrificing the operation bandwidth and linearity.
It is a still further object of the present invention to provide an integrated dielectric light funnel right in front of the detector""s coupling facet for protecting the facet.
Besides the objects described above, the present invention also has the advantage of making a high-density edge-coupled photodetector array with relaxed alignment tolerance due to the enlarged coupling aperture. The arrayed photodetectors are particularly useful in the applications with a high data volume, such as ultrahigh-speed parallel data communication or wavelength division multiplexing (WDM) systems. Since each photodetector can achieve high-speed operation with high receiving efficiency, such array can manipulate a data volume of several tens or even several hundreds of Gigabits per second.